The invention relates to a progressive spectacle lens according to the preamble of patent claim 1, the lens exhibiting an only small change of binocular properties during a movement of glance.
Progressive spectacle lenses (also called varifocal lenses, multifocal lenses etc.) are usually understood to be spectacle lenses having a different (lower) power in the region through which a spectacles wearer views an object located at a great distancexe2x80x94hereunder referred to as a distance portionxe2x80x94than in the region (near portion) through which the spectacles wearer views a near object. Located between the distance portion and the near portion is the so-called progressive zone in which the power of the spectacle lens continuously increases from that of the distance portion to that of the near portion. The magnitude of the power increase is also designated as addition power (Add).
As a rule, the distance portion is located in the upper part of the spectacle lens and designed for viewing xe2x80x9cto infinityxe2x80x9d, whilst the near portion is located in the lower region and is particularly designed for reading. In spectacles for special applicationxe2x80x94those for pilots or for monitor work stations are mentioned as examplesxe2x80x94the distance and near portions may also be arranged differently and/or designed for other distances. Furthermore, it is possible for a plurality of near portions and/or distance portions and suitable progressive zones to be present.
With progressive spectacle lenses having a constant refractive index it is necessary, in order that the power may increase between the distance portion and the near portion, that the curvature of one or both surfaces continuously change from the distance portion to the near portion.
The surfaces of spectacle lenses are usually characterized by the so-called principal radii of curvature R1 and R2 at every point on the surface. (Sometimes also the so-called principal curvatures K1=1/R1 and K2=1/R2 are given instead of the principal radii of curvature.) Together with the refractive index n of the glass material, the principal radii of curvature govern the parameters frequently used for an ophthalmologic characterization of a surface:
Surface power=0.5xc2x7(nxe2x88x921)xc2x7(1/R1+1/R2) 
Surface astigmatism=(nxe2x88x921)xc2x7(1/R1xe2x88x921/R2) 
Surface power D is the parameter via which an increase of power from the distance portion to the near portion is achieved. Surface astigmatism (more clearly termed cylinder power) is a xe2x80x9ctroublesome propertyxe2x80x9d, because an astigmatismxe2x80x94inasmuch as an eye does not have an innate astigmatism to be correctedxe2x80x94that exceeds a value of about 0.5 dpt results in an indistinctly perceived image on the retina.
Although any change of the curvature of the surface, as needed to achieve a surface power increase without vision being xe2x80x9cdisturbedxe2x80x9d by surface astigmatism, can be attained relatively simply along a (plane or winding) line, considerable xe2x80x9cintersectionsxe2x80x9d of surfaces will result alongside this line, leading to a large surface astigmatism which more or less impairs the lens in regions alongside the mentioned line.
A further consequence of these intersections is that the spectacle lens will have different properties at respective see-through positions on the nasal and temporal side. Binocular vision, in particular, may be adversely affected thereby. This becomes noticeable mainly during movements of glance and is therefore disturbing:
When a spectacles wearer allows his gaze to follow a moving object whilst keeping his head at rest, his visual impression will depend, on the one hand, on the imaging quality of both lenses of his spectacles at the positions he sees through when making the necessary eye movements to follow the object. When the spectacles wearer encounters small image defects (astigmatism, refraction defects, etc.) at these see-through positions, he will see the object more distinctly than when large image defects are present.
On the other hand, however, as a rule both eyes participate in seeing, and the total visual impression will be composed of the visual impressions of both eyes.
Thus, it will be of consequence, for example, whether the object can be binocularly perceived as a single object, how large the effort of fusion is, and whether the spectacles wearer can see the object equally distinctly with both eyes, or well with one eye and badly with the other eye. Finally, in the case of moving objects, any changes of this binocular visual impression whilst the object is being followed will be of importance.
The invention is based on the object of further developing a progressive spectacle lens of the kind set out in the preamble of claim 1 in such manner that the optical parameters that are relevant to the quality of the binocular visual impression will change as little as possible when the glance is being shifted.
The achievement of this object in accordance with the invention is set out in patent claim 1. Further developments of the invention are the subject matter of the dependent claims.
According to the invention it has been realized that the binocular properties that are relevant to an achievement of the set object are the astigmatic difference, the refraction equilibrium, and the vertical prismatic deviation.
These parameters are obtained by computing the principal ray from the centre of rotation of the right eye through a point on the front surface of the right-hand spectacle lens to the object point, and the associated wave front. From the data of this wave front and the prescription for the right eye, the astigmatic deviation and the refraction error are computed in the generally known manner. Subsequently the principal ray and the wave front from the object point through the centre of rotation of the left eye are iterated, assuming intersecting visual axes (orthotropy).
From this, the corresponding see-through points on the right-hand and the left-hand lens have been computed.
The astigmatic deviation and the refraction defect of the wave front through the left-hand spectacle lens are combined with the values for the right-hand lens and thus provide the parameters of astigmatic difference (according to the method of obliquely crossed cylinders) and refraction equilibrium (absolute value of the difference between the mean powers of the spectacle lenses). The vertical prismatic deviation is obtained by projecting the eye-side principal rays onto the Cyclops eye plane and expressing the angle between the straight lines in cm/m.
Of interest for eye movements performed in reading or following moving objects are not only the magnitudes of these parameters, but also their changes during a movement. These changes may be approximately characterized by their xe2x80x9cliftxe2x80x9d, i.e. the difference between the maximum and the minimum values occurring with the movement.